U.S. Pat. No. 5,638,423 teaches activating a wireless radio terminal, such as a cellular phone, in response to a signal received from a proximity user card over a short-range RF (Radio Frequency) link. The wireless terminal is activated for use when the wireless terminal and the proximity user card are within a predetermined distance from one another, which aids in preventing unauthorized use of the wireless terminal. The predetermined distance is set by the transceiving range between the proximity user card and the wireless terminal. However, a disadvantage of the method taught by U.S. Pat. No. 5,638,423 is that it depends on signal amplitude of a low power, short-range RF link between the wireless terminal and the proximity user card, which is inherently unreliable.
The wireless terminal transmits an inquiry signal, via the low power, short-range RF link, to the card. If the wireless terminal's inquiry to the card goes unanswered, logic in the wireless terminal determines that the wireless terminal and the card are separated by a distance greater than the predetermined distance, and the wireless terminal transmits an invalid proximity message and is kept locked. Since the range and reliability of a low power, short-range RF link varies according to signal loss imposed by its environment, the method of activating a wireless terminal for use as taught in U.S. Pat. No. 5,638,423 is subject to erroneous determinations.
The disclosed method overcomes one or more of the abovementioned problems.